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Taikun OCP Guide

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Oversubscription in thin provisioning

OpenStack Block Storage enables you to choose a volume back end based
on virtual capacities for thin provisioning using the oversubscription

A reference implementation is provided for the default LVM driver.
The illustration below uses the LVM driver as an example.

Configure oversubscription

To support oversubscription in thin provisioning, a flag
max_over_subscription_ratio is introduced into
cinder.conf. This is a float representation of the
oversubscription ratio when thin provisioning is involved. Default ratio
is 20.0, meaning provisioned capacity can be 20 times of the total
physical capacity. A ratio of 10.5 means provisioned capacity can be
10.5 times of the total physical capacity. A ratio of 1.0 means
provisioned capacity cannot exceed the total physical capacity. A ratio
lower than 1.0 is ignored and the default value is used instead.

This parameter also can be set as
max_over_subscription_ratio=auto. When using auto, Cinder
will automatically calculate the
max_over_subscription_ratio based on the provisioned
capacity and the used space. This allows the creation of a larger number
of volumes at the beginning of the pool’s life, and start to restrict
the creation as the free space approaches to 0 or the reserved


max_over_subscription_ratio can be configured for each
back end when multiple-storage back ends are enabled. It is provided as
a reference implementation and is used by the LVM driver. However, it is
not a requirement for a driver to use this option from

max_over_subscription_ratio is for configuring a back
end. For a driver that supports multiple pools per back end, it can
report this ratio for each pool. The LVM driver does not support
multiple pools.

Setting this value to ‘auto’. The values calculated by Cinder can
dynamically vary according to the pool’s provisioned capacity and
consumed space.

The existing reserved_percentage flag is used to prevent
over provisioning. This flag represents the percentage of the back-end
capacity that is reserved.


There is a change on how reserved_percentage is used. It
was measured against the free capacity in the past. Now it is measured
against the total capacity.


Drivers can report the following capabilities for a back end or a

thin_provisioning_support = True(or False)
thick_provisioning_support = True(or False)
provisioned_capacity_gb = PROVISIONED_CAPACITY
max_over_subscription_ratio = MAX_RATIO

Where PROVISIONED_CAPACITY is the apparent allocated
space indicating how much capacity has been provisioned and
MAX_RATIO is the maximum oversubscription ratio. For the
LVM driver, it is max_over_subscription_ratio in

Two capabilities are added here to allow a back end or pool to claim
support for thin provisioning, or thick provisioning, or both.

The LVM driver reports thin_provisioning_support=True
and thick_provisioning_support=False if the
lvm_type flag in cinder.conf is
thin. Otherwise it reports
thin_provisioning_support=False and

Volume type extra specs

If volume type is provided as part of the volume creation request, it
can have the following extra specs defined:

'capabilities:thin_provisioning_support': '<is> True' or '<is> False'
'capabilities:thick_provisioning_support': '<is> True' or '<is> False'


capabilities scope key before
thin_provisioning_support and
thick_provisioning_support is not required. So the
following works too:

'thin_provisioning_support': '<is> True' or '<is> False'
'thick_provisioning_support': '<is> True' or '<is> False'

The above extra specs are used by the scheduler to find a back end
that supports thin provisioning, thick provisioning, or both to match
the needs of a specific volume type.

Volume replication extra

OpenStack Block Storage has the ability to create volume replicas.
Administrators can define a storage policy that includes replication by
adjusting the cinder volume driver. Volume replication for OpenStack
Block Storage helps safeguard OpenStack environments from data loss
during disaster recovery.

To enable replication when creating volume types, configure the
cinder volume with
capabilities:replication="<is> True".

Each volume created with the replication capability set to
True generates a copy of the volume on a storage back

One use case for replication involves an OpenStack cloud environment
installed across two data centers located nearby each other. The
distance between the two data centers in this use case is the length of
a city.

At each data center, a cinder host supports the Block Storage
service. Both data centers include storage back ends.

Depending on the storage requirements, there can be one or two cinder
hosts. The administrator accesses the
/etc/cinder/cinder.conf configuration file and sets
capabilities:replication="<is> True".

If one data center experiences a service failure, administrators can
redeploy the VM. The VM will run using a replicated, backed up volume on
a host in the second data center.

Capacity filter

In the capacity filter, max_over_subscription_ratio is
used when choosing a back end if thin_provisioning_support
is True and max_over_subscription_ratio is greater than

Capacity weigher

In the capacity weigher, virtual free capacity is used for ranking if
thin_provisioning_support is True. Otherwise, real free
capacity will be used as before.